In Geetaa 18th Chapter, Shlok 67 and 68 Krishn gives a description outlining to whom this knowledge should not be given and to whom it should be given.
In Bhaagavat - in Kapilopadesh, Kapil Jee also mentions that those who are not qualified for this knowledge and those who are qualified. Here is the list:

This knowledge should not be given to:
1. Those who are wicked
2. Those who are arrogant
3. Those who are stubborn
4. Those who have bad conduct (A-Dhaarmik)
5. Those whose are hypocrites
6. Those who are indulging excessively in sense objects
7. Those who are excessively possessive of home etc
8. Those who are not devoted to Me (higher nature)
9. Those who are disrespectable to or hate My devotees (or disrespect to those who are in this higher path)

This knowledge should be given to:
1. Those who have strong faith (Shraddhaa) towards higher nature - the Ved and the teacher.
2. Those who are My devotees
3. Those who are humble
4. Those who are never jealous of others
5. Those who are friends to all living beings
6. Those who revel in serving others
7. Those who have dispassion to all external possessions.
8. Those who are in peace with themselves
9. Those who do not envy
10. Those who are pure
11. Those who respects those who are devoted to Me.

Who makes this decision? Of course the teacher. In olden days the teacher used to observe the student who has approached him for knowledge for a year or so before he decides to impart that knowledge.

In principle - like any other science, there are pre-requisites for any kind of knowledge. Those who do not have them should not get it even if teacher gives!